1. Field of the Invention
This invention relates to a target tracking system and more particularly, to an improvement of target tracking system for tracking a target with a television camera moved based on the movement of the target.
2. Description of the Related Art
Heretofore, there has been provided a target tracking system that tracks a target with a television camera moved based on the movement of the target. In this case, the system detects the movement of the target in an image imaged by the television camera. Block matching is commonly used to detect movement. In block matching, the image around the target or a partial image characterizing the target is set as an original block, and the subsequent image is divided into blocks to detect the block having the minimum sum of absolute differences between pixels of the original block and pixels of the tested block, and the location of the target is determined.
However, in a target tracking system using block matching, a problem arises in that, because the location of a target is determined based on shape information included in the original block, tracking becomes difficult if the target itself changes shape or size. An additional problem arises in that, because calculating the minimum sum of absolute differences between pixels in the original block and those in the block in an image requires a large amount of calculation, the circuit structure becomes complicated, and calculation using a microcomputer becomes difficult.
To solve these problems, a method can be considered that sets a fixed color in color image signals as a characteristic quantity and that tracks each region that has horizontally and vertically integrated values of a color image corresponding to the color in a time continuum with previous regions, namely, regions adjacent to previous regions as a target. However, in this case, a problem arises in that it is difficult to determine delicate differences and changes in hue due to setting of a characteristic color and to determine location due to broken continuity in time-space images if a region is distant, namely, the target is small, or the target moves at comparatively high speed. An additional problem arises in that it is necessary to store a number of screens of the time-space image, which increases memory required for image storage.
A method can be considered that tracks a target, evaluating horizontal and vertical directions of a color image, and another method can track the target, evaluating horizontal and vertical directions from the error between actual data and that calculated from previous and current forecast coefficients, in the case of a set color, after the method sets the color and sets the forecast coefficient calculated from tint and saturation of the set color as a characteristic quantity.
However, since all of these methods set hue (the angle formed by R-Y and B-Y) as the characteristic quantity, it is difficult to track a target in the case of hue changes due to small changes in R-Y or B-Y and, as these methods get information mainly from hue, it is difficult to track a target which changes its hue based on the luminance of black and white illumination. In addition, an additional problem arises in that, since almost no television camera outputs saturation or hue signals, transformation must be made from color difference signals, complicating the structure.
Also, there has been provided a target tracking system that automatically tracks a target by utilizing the difference between fields of image (Japanese Patent Publication No.056111/1975). In the target tracking system, the inter-fields difference is evaluated per pixel, and a pan and tilt motor is controlled in accordance with the position of pixel that fluctuation is detected and the difference of the distance from the center.
However, in the target tracking system described above, there are some problems that the system works improperly due to level fluctuation caused by image signal noise because the inter-fields difference is evaluated per pixel, the moving object may exist in the screen other than the target area, and random fluctuation in the image may occur while the television camera is improperly operated.
Also, there has been provided a target object tracking system that displays a frame indicating the moving object to be tracked in an image, and tracks the frame by a video camera in accordance with the detected result of the motion vectors of a plurality of blocks in the frame (European Patent Publication No.0557007).